Mac: With Activity Monitor, you can quickly see which of your apps is hogging system resources and shut it down when need be, but Mac OS X Hints points out that now in Mavericks you can get even more granular and see which tab in an app like Safari is eating that CPU.

All you need to do is open up Activity Monitor and look for "Safari Web Content." Hover over that item with your mouse, and it'll display the URL in the that tab. It's a pretty easy way to identity which Safari tab is hogging up your CPU.